1. Field of the Invention
The present invention relates to an automatic transmission for an automobile and more particularly to an automatic transmission whose friction type starting clutch and forward and reverse changeover apparatus are packaged in a sealed clutch housing.
2. Prior Art
Generally, an automatic transmission having a continuously variable transmission comprises a starting apparatus for controlling a power transmission from an engine to a transmission on starting, a forward and reverse changeover apparatus for reversing the rotational direction of a drive shaft in order to propel a vehicle in the reverse direction and a clutch section for changing over driving force supplied to the forward and reverse changeover apparatus.
FIG. 3 shows a skeleton of an example of an automatic transmission 1 in which a starting apparatus and a forward and reverse changeover apparatus are arranged on the input side of the continuously variable transmission. A driving force of an engine 25 is transmitted to a planetary carrier 4b supporting a double planetary pinion 4a of a planetary gear 4 through a damper unit 2 secured to an output shaft 26 of the engine 25.
At starting forward, a reverse brake plate 5 which is connected with a ring gear 4c of the planetary gear 4 is released and a forward clutch plate 6 which is connected with the planetary carrier 4b is gradually engaged by hydraulic pressure supplied to a clutch operating chamber 6a. In this example, the forward clutch plate 6 also has a function as a starting clutch. When the forward clutch plate 6 is fully engaged, the driving force of the engine 25 is transmitted to a primary pulley 7a through a plunger 7g connected with the primary pulley 7a, whereby the primary pulley 7a is rotated in the right direction.
At reverse running, the forward clutch plate 6 is released and the reverse brake plate 5 is gradually engaged by hydraulic pressure supplied to a brake operating chamber 5a. In this example, the reverse brake plate 5 is employed also for a starting clutch. When the reverse brake plate 5 is fully engaged, the ring gear 4c is fixed and the planetary pinion 4a rotates a sun gear 4d as provided on an input shaft 7c formed integrally with the primary pulley 7b at a specified reduction speed in the reverse direction. In this drawing, numeral 8 denotes a flywheel and numeral 9 denotes an oil pump.
Further, FIG. 4 shows another example of an automatic transmission 1 in which a continuously variable transmission 7 is arranged on the input side of the forward and reverse changeover apparatus and a friction type starting clutch is arranged on the output side thereof. A driving force of the engine 25 is inputted to a planetary input shaft 10 through the damper unit 2 secured to the output shaft 26 of the engine 25. The planetary input shaft 10 is connected with a clutch hub 4e and the sun gear 4d of the planetary gear 4.
At starting forward, the reverse brake plate 5 provided on the outer periphery of the ring gear 4c is released and on the other hand the forward clutch plate 6 connected with the planetary carrier 4b is engaged. The driving force inputted to the planetary input shaft 10 is transmitted to the input shaft 7c formed integrally with the primary shaft 7b secured to the primary pulley 7a of the continuously variable transmission 7, thereby the primary pulley 7a is rotated in the right direction.
Further, at reverse running, the forward clutch plate 6 is released and the reverse brake plate 5 is engaged. When the reverse brake plate 5 is engaged, the ring gear 4c is fixed and the driving force inputted to the planetary input shaft 10 is transmitted to the input shaft 7c through the sun gear 4d, the planetary pinion 4a and the planetary carrier 4b supporting this planetary pinion 4a, thereby the input shaft 7c is rotated at a reduced speed in the reverse direction.
At starting forward or in the reverse direction, when the driving force is transmitted to the primary pulley 7a, a secondary pulley 7e interconnected with the primary pulley 7a through a belt 7d rotates. At starting, a friction type starting clutch 11 connected with an output shaft 7f of the secondary pulley 7e is gradually engaged and the driving force is transmitted to drive shafts 14 interconnected with front or rear wheels through the friction type starting clutch 11, a final reduction gear unit 12 and a differential gear 13. The oil pump 9 is driven by the planetary input shaft 10 through a chain 15.
For example, Japanese Patent Application Laid-open No. Toku-Kai-Hei 4-165149 discloses a technique in which the friction type starting clutch 11 is disposed on the output side of the secondary pulley 7e of the continuously variable transmission.
FIG. 5 shows a case where the engine 25 and the automatic transmission 1 are arranged transversely, in which an overall width W.sub.S of the combination of the engine 25 and the automatic transmission 1 must be of a size capable of accommodating the combination in an engine room 21a.
Recent motor vehicles have a frame 22 on both sides of the engine room 21a respectively in order to absorb an impact in case the vehicle has a collision. Furthermore, a front tire 23 is arranged outside of the frame 22, respectively.
The frame 22 must have a certain amount of sectional area sufficient to have rigidity for securing a safety on impact and also the front tire 23 is required to move within as large steering angle as possible in order to obtain a small turning circle radius. Further, recent motor vehicles have a tendency to reduce the width of the vehicle body due to a necessity of improving fuel economy.
As a result of this, a space for accommodating the engine 25 and the transmission 1 in the engine room 21a tends to decrease, therefore the overall width W.sub.s of the combination of the engine and transmission is required to be shortened as far as possible. One idea for realizing this is to reduce a widthwise size W.sub.T of the automatic transmission 1 itself.
That is, in order to reduce the size W.sub.T in the axial direction of the automatic transmission 1 arranged in this manner, several means such as decreasing the pulley ratio of the continuously variable transmission, decreasing the spline-fitting width of the planetary input shaft 10, reducing the wall thickness of components, closing clearances between components or the like can be considered. However, any of these means may adversely affect the function of the automatic transmission 1 or may reduce the rigidity and strength of the automatic transmission 1.
On the other hand, since the speed reduction ratio of the continuously variable transmission varies continuously, it is connected with the engine directly. Generally, in case of a multi-stage automatic transmission, it is necessary to generate a slip leading to a power loss in the clutch in order to alleviate a shift shock. The continuously variable transmission is free from such a power loss. Further, in the continuously variable transmission, since it is possible to maintain the speed reduction ratio in an optimum state in the relationship between vehicle and engine speeds, a driving efficiency is largely improved and as a result fuel economy and running performance can be enhanced.
In general, it is necessary to enlarge a controllable range of the speed reduction ratio in order to raise a driving efficiency. Further, inherently, the starting apparatus acts as shutting off the power transmission between the engine and the transmission when the engine revolution can be maintained no more due to an excessive drop of the engine speed in such cases as stopping or starting or when the direction of power transmission changes in such a case as changing-over from forward to reverse or vice versa. Therefore, since the starting apparatus may be adequate if this shutting-off function is satisfied on starting, stopping or changing-over the running direction, there is a leeway to reduce the size of the starting apparatus itself.
However, on the other hand, there is a case where the driving force must be transmitted with the starting apparatus kept in a slip state when a large driving force is needed in such a case as starting in creep. The starting apparatus held in this state generates a large amount of heat and this results in a seizure in the clutch facing. The seizure incurs not only a reduced performance of power transmission but also a short life of the starting apparatus.
With respect to the automatic transmission shown in FIG. 3, the forward clutch plate 6 and the reverse brake plate 5 are both used also for a starting apparatus but these clutch and brake plates should be used inherently for clutching or braking, respectively, not for a starting apparatus capable of withstanding heavy duty. Accordingly, these components are lacking in heat capacity endurable in heat generation at consecutive stall startings.
Further, with respect to the automatic transmission shown in FIG. 4, since the friction type starting clutch 11 is disposed on the output side of the secondary pulley 7e which is subjected to a larger torque than the input side of the primary pulley 7a, some means for increasing a heat capacity are required.
As an example of a technique having a heat countermeasure, there is Japanese Patent Application Laid-open No. Toku-Kai-Hei 3-219121, in which the clutch plate of the friction type starting clutch is accommodated in a clutch cover wherein oil is circulated to cool the clutch facing. According to this prior art, however, the clutch cover needs a certain amount of volume for accommodating the clutch plate therein and as a result this elongates an axial length of the automatic transmission.